Criminals could have stolen the bank and credit card details of more than four million people after a "significant and sustained" cyber attack on TalkTalk.

Police are now investigating after the broadband and phone provider reported it had been hacked on Wednesday.

The Metropolitan Police's Cyber Crime Unit is now investigating.

TalkTalk said it was now contacting customers to inform them they may be at risk.

The company also said the hackers may have accessed dates of birth, telephone numbers, names and addresses of its clients.

In a statement on its website, the company's managing director Tristia Harrison said: "We are very sorry to tell you that on Thursday 22nd October a criminal investigation was launched by the Metropolitan Police Cyber Crime Unit following a significant and sustained cyberattack on our website on Wednesday 21st October.

"We are continuing to work with leading cyber crime specialists and the Metropolitan Police to establish exactly what happened and the extent of any information accessed.

We would like to reassure you that we take any threat to the security of our customers’ data very seriously.

"Unfortunately cyber criminals are becoming increasingly sophisticated and attacks against companies which do business online are becoming more frequent."

The company said the website had now been made secure and it had contacted all the major banks which would be monitoring accounts for suspicious activity.

It warned customers to keep an eye on their accounts over the next few months and report anything unusual to the bank and Action Fraud as soon as possible.

It also urged customers to check their credit report.

TalkTalk reassured customers it would never ask for a password or bank details without customers' permission.

Ms Harrison added: "We understand this will be concerning and frustrating, and we want to reassure you that we are continuing to take every action possible to keep your information safe."

Dido Harding, CEO, said: “TalkTalk constantly updates its systems to make sure they are as secure as possible against the rapidly evolving threat of cyber crime, impacting an increasing number of individuals and organisations.

"We take any threat to the security of our customers’ data extremely seriously and we are taking all the necessary steps to understand what has happened here. As a precaution, we are contacting all our customers straight away with information, support and advice around yesterday’s attack.”

The Metropolitan Police confirmed its Cyber Crime Unit was investigating an allegation of data theft which was reported on Wednesday.

"There have been no arrests and enquiries are ongoing," a force spokeswoman said.

The Talk Talk website was unavailable to customers tonight and displayed the message: "Sorry we are currently facing technical issues, our engineers are working hard to fix it.

"We apologise for any inconvenience this may cause."

Anyone who thinks they may be a victim of fraud should contact their bank and ring up Action Fraud, the UK’s national fraud and internet crime reporting centre on 0300 123 2040 or email www.actionfraud.police.uk